does 3d come before 4s?

[unsung]

I thought 4s comes before 3d, but why is the ground state configuration for titanium given as [Ar] 3d2 4s2? Shouldn't it be [Ar] 4s2 3d2?

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[RPedigo]

4s does get filled before 3d when building up electrons, but they just rearranged it by increasing energy (since the number before the letter is the principal quantum number n, which denotes energy level). So since 3 is less than 4, they just re-ordered it. Remember that if you tried to remove electrons from that element the 4s electrons would leave first since it has the higher principal quantum number. They're just making that more explicit by putting the highest energy electrons last.

 

[unsung]

4s does get filled before 3d when building up electrons, but they just rearranged it by increasing energy (since the number before the letter is the principal quantum number n, which denotes energy level). So since 3 is less than 4, they just re-ordered it. Remember that if you tried to remove electrons from that element the 4s electrons would leave first since it has the higher principal quantum number. They're just making that more explicit by putting the highest energy electrons last.

I thought if it gets filled up first that means it's lower in energy? Maybe I'm confusing myself.

 

[RPedigo]

I thought if it gets filled up first that means it's lower in energy? Maybe I'm confusing myself.

So if you look at the periodic table, you have elements like potassium and calcium which have 4s electrons. Then afterward you go into the transition metals which have 3d electrons. So you do fill the 4s before the 3d. However, when you remove electrons, you need to remove the 4s electrons before the 3d electrons, since you remove from the highest principal quantum number first. (If they share the same principal quantum number you remove the one with the highest l value first, meaning remove 3p electrons before 3s electrons.)

So scandium is [Ar]4s(2) 3d(1) or you can rearrange it in energetic order afterward and put it as [Ar]3d(1) 4s(2). They mean the same thing. Now if you have Sc++, you actually have [Ar]3d(1) 4s(0) since 4s has a higher principal quantum number than 3d. It is confusing, but with practice you'll get the hang of it 👍

The principal quantum number refers to energy and therefore distance. The 4s orbital is further from the nucleus than the 3d orbital. Even though the 3d electrons don't penetrate well, they're still closer to the nucleus than the 4s electrons and are therefore not removed until the 4s electrons go away.

 

[unsung]

So if you look at the periodic table, you have elements like potassium and calcium which have 4s electrons. Then afterward you go into the transition metals which have 3d electrons. So you do fill the 4s before the 3d. However, when you remove electrons, you need to remove the 4s electrons before the 3d electrons, since you remove from the highest principal quantum number first. (If they share the same principal quantum number you remove the one with the highest l value first, meaning remove 3p electrons before 3s electrons.)

So scandium is [Ar]4s(2) 3d(1) or you can rearrange it in energetic order afterward and put it as [Ar]3d(1) 4s(2). They mean the same thing. Now if you have Sc++, you actually have [Ar]3d(1) 4s(0) since 4s has a higher principal quantum number than 3d. It is confusing, but with practice you'll get the hang of it 👍

The principal quantum number refers to energy and therefore distance. The 4s orbital is further from the nucleus than the 3d orbital. Even though the 3d electrons don't penetrate well, they're still closer to the nucleus than the 4s electrons and are therefore not removed until the 4s electrons go away.

Thanks!!